Oxetanes, which are also known as oxacyclobutanes or trimethylene oxides, are of interest as intermediates in organic chemistry by virtue of their reactivity and their relatively low toxicity in comparison with epoxides. They may, for example, be used as starting substances for polymers or in radiation curing (U.S. Pat. No. 5,750,590, U.S. Pat. No. 5,463,084) or in the thermal crosslinking of powder coatings (WO 9719138).
Di-, tri- or polyfunctional oxetanes are frequently used for these purposes. These substances are generally synthesized from hydroxy-functionalised oxetanes. Hydroxyfunctional oxetanes, such as for example 3-ethyl-hydroxymethyloxetane, are readily obtainable from trimethylol compounds, such as for example trimethylolpropane. The oxetanes are produced be reacting the trimethylol compounds with phosgene (DE 972508), diaryl carbonates, ethylene carbonate (JP 10007669), chloroformates (DE 972209) or dialkyl carbonates (D. B Pattison, Am. Soc., 79, (1957), 3456).
Since production using phosgene and diphenyl carbonate is problematic due to the toxic properties of phosgene and phenol, production using alkyl carbonates, such as diethyl carbonate, provides a good alternative. In this production method, the dialkyl carbonate is mixed with the trimethylol compound. A basic catalyst is added to the mixture and the trimethylol compound is transesterified. In order efficiently to separate the alcohol arising during the transesterification from the as yet unreacted dialkyl carbonate and thus to complete the reaction, an efficient column is used in most processes. In another process, the alcohol produced is continuously removed by distillation without a column together with as yet unreacted dialkyl carbonate, wherein a large excess of dialkyl carbonate is used (PCT/EP97/00829). This method is economically disadvantageous because the distillation mixture must be worked up or disposed of. After the transesterification, the intermediate, which has not been isolated and comprises cyclic carbonates and oligocarbonates, is generally deoligomerised and decarboxylated. This operation proceeds at elevated temperatures of 160-210.degree. C. in the presence of basic catalysts (U.S. Pat. No. 5,463,084, Example 1, U.S. Pat. No. 5,721,020, page 8, U.S. Pat. No. 5,750,590, Example 1, Kurt C. Frisch in Cyclic Monomers, Wiley Intersciences, New York, 1972, page 70). The disadvantage of these relatively high reaction temperatures is that, when the products spend an extended period in the reactor, for example in the case of large batches, secondary products, in particular acroleins, such as 2-ethylacrolein, may be formed. Oxetanes are known to decompose into unsaturated carbonyl compounds (Houben-Weyl, Methoden der Organischen Chemie, volume 6/3, page 509) and functionalised oxetanes are particularly susceptible to such decomposition. These decomposition products, however, contribute to a change in the odour of the products. If the product contains small quantities of these secondary products, the products, which are per se odourless, take on, for example, an odour like coconut milk or an aromatic/sweetish odour, which is also mentioned in the manufacturers' corresponding safety data sheets. A larger proportion of secondary products accordingly results in a marked odour nuisance. This is undesirable for certain applications, such as for example in radiation curing. Accordingly, in order to increase product purity, it is necessary to perform an additional special distillation which further purifies the product (D. B. Pattison, Am. Soc., 79 (1957), page 3456, U.S. Pat. No.5,721,020, column 14, U.S. Pat. No. 5,750,590, Example 1, U.S. Pat. No. 5,436,084, Example 1).